Knowing how to negotiate =/= agreeing to be ok with something before you're actually ok with it, which is the only thing that I see that you could have done differently. In the end you didn't even try to restrict her, you said she could go ahead and have sex with her secondary, you just added the caveat that you wouldn't be okay with it and would need some time. That's called honesty. Sacrificing honesty for the sake of giving her what she wanted wouldn't have been compromise or negotiation, it would simply have been emotional falsehood.
The fact that she didn't live up to what you asked of her doesn't mean it was too much to ask. It just means that she screwed up (and then massively compounded it by lying). It's on her to do the work to fix it and become a better partner, if indeed she wants to.
It seems to me like you're trying to find ways to shift some of the blame for this situation on to yourself. And I get that there are two sides to every story, but I'm just not seeing where you messed up here. What if you actually didn't? Why, then, are you invested in figuring out how you did? Maybe it's a function of the low self-esteem thing, like it just feels natural to believe that you must be at least in part at fault if something has gone wrong. Or maybe it's about the totally understandable desire to think well of your partner. Just some things to think about.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.